The single most tested differentiator: surgery is curative in UC, never curative in Crohn's. Transmural involvement in Crohn's explains fistulae and abscesses; mucosal-only UC explains why toxic megacolon is its major acute complication. PSC (primary sclerosing cholangitis) is strongly linked to UC, not Crohn's.
Key NEET trap: sigmoid volvulus first-line = endoscopic decompression (not immediate surgery). Surgery (Hartmann's procedure) is reserved for gangrenous bowel or failed decompression. Recurrence after decompression alone is ~50%, so elective sigmoid resection follows once the patient is optimised. Caecal volvulus almost always requires surgery.
MANTRELS mnemonic maps to the Alvarado score (max 10). RIF tenderness (2 pts) and leukocytosis (2 pts) carry double weight. Score ≥7 = proceed to surgery without imaging in most settings. In women of reproductive age, USS (or MRI in pregnancy) is recommended even with high scores to exclude gynaecological pathology. The negative appendicectomy rate should ideally be <20%.